It is known to use fluid sensors to measure the composition and/or flow characteristics of a fluid. Such fluid sensors are often referred to as multiphase meters. Known multiphase meters comprise a base pipe defining a fluid flow path internally thereof surrounded by a concentrically arranged open-ended generally cylindrical metallic cavity member. The base pipe is substantially transparent to radio frequency (RF) electromagnetic radiation. The cavity member defines a cavity for a RF electromagnetic field which extends through the base pipe and across the fluid flow path. In known multiphase meters the base pipe may be formed of polyvinyl chloride (PVC) or polyether ether ketone (PEEK) and the cavity member is formed of brass. Such known multiphase meters are configured to detect a resonant peak in the frequency spectrum of the RF electromagnetic field and to extract the composition and/or flow characteristics of fluid in the fluid flow path from characteristics of the resonant peak.
It is well known that the strength of an RF electromagnetic field varies across a resonant cavity. Consequently, when a non-homogeneous fluid is present in the fluid flow path, different fluid components (e.g. water, oil or gas) present in the fluid may be located or flow through regions having significantly different RF electromagnetic field strengths. If the different fluid components move position across the fluid flow path this can make measurements of the composition and/or flow characteristics of the fluid in the fluid flow path more difficult and/or less accurate. Accordingly, in known multiphase meters, the cavity member is generally separated from the base pipe so as to define a resonant cavity which is significantly greater in cross-section than the fluid flow path for improved uniformity of the RF electromagnetic field strength across the fluid flow path. Consequently, known multiphase meters have an annular outer cavity region defined between an outer surface of the base pipe and an inner surface of the cavity member.
In known multiphase meters the annular outer cavity region is filled with air or water. Examples of such known multiphase meters are described in S. Al-Hajeri, S. R. Wylie, R. A. Stuart and A. I. Al-Shamma'a, “An electromagnetic cavity sensor for multiphase measurement in: the oil and gas industry”, Journal of Physics: Conference Series 76 (2007) 012007; in S. Al-Hajeri, S. R. Wylie, A. Shaw and A. I. Al-Shamma'a “Real time EM waves monitoring system for oil industry three phase flow measurement”, Journal of Physics: Conference Series 178 (2009) 012030; in S. R. Wylie, A. I. Al-Shamma'a, A. Shaw and S. Al-Hajeri, “Electromagnetic cavity sensors for multiphase measurement”, Exploration and Production Oil and Gas Review, Volume 9, Issue 1; and in Finnish patent document no. F1834892.
The use of a fluid sensor comprising a brass cavity member and an air-filled outer cavity region may be problematic especially in a high pressure environment because the brass cavity member may have to withstand high external pressures. This may require the use of a relatively thick brass cavity member. Alternatively, the cavity member may be surrounded by a protective external casing, for example a steel external casing which is configured to withstand high external pressures. This may require the use of a relatively thick external casing. Known brass cavity members may also be susceptible to erosion and/or corrosion in subsea environmental conditions or in the environmental conditions of an oil or gas well. Use of an external casing may also be necessary to protect a brass cavity member from erosion and/or corrosion in subsea environmental conditions or in the environmental conditions of an oil or gas well.
The use of a fluid sensor comprising a brass cavity member and a water-filled outer cavity region may also be problematic because, even though water is generally much less compressible than air, if the external fluid pressure is sufficiently high, it may still be necessary for the brass cavity member and/or an external casing to be configured to withstand high external fluid pressures.